The invention relates to a apparatus for investigating thermal transformation and to a quasi-static temperature control system for the investigation of phase transformations using a differential thermal analyser (DTA) or differential scanning calorimeter (DSC). Quasi-static temperature means that the temperature is increases continuously until thermal transformation has taken place, or conversely it means that the transformation should proceed at the same temperature under static circumstances.
Chemical and physical changes taking place as a consequence of heating involve the change of enthalpy that results in a variation of temperature of the investigated sample different from that of the ambiency. Differential thermal analyzers (DTA) have been used for the investigation of such processes for almost a century. Recently differential scanning calorimeters (DSC) are also used for the same purposes.
The DTA apparatus operates by measuring the difference between the temperature of a sample in a sample containerette and of a reference substance with similar thermal parameters. A temperature difference will only appear when the transformation in the sample begins, that is, a change of enthalpy takes place in the sample. During the investigation, both the sample and the reference substance are placed in a furnace, the temperature of which is increased with a predetermined speed by a program-controller and the temperature of sample and of reference substance as well as the difference of temperature between them is being measured and recorded. Nevertheless the, known DTA apparatus do not permit the determination characteristic temperatures belonging to individual transformations with due accuracy, which would be of very great imprtance with respect of identification of the transformation, since the temperature of the furnace grows continuously and thus the transformation takes place in a relatively wider range of temperature.
Our intention by means of the invention has been to work out a method to investigate thermal transformation and a quasi-static temperature controlling system for the investigation of phase transformations, where the phase transformation itself, controlling the temperature of the furnace, stop the increase of temperature from its beginning up to its coming to an end. At the same time the phase transformation itself ensures that the difference between the temperature of the sample and of the furnace may only be so high at which the sample may take up an amount of heat from the furnace necessary to a uniform and very low speed phase transformation.
The structure of the temperature-controller is based on the recognition that the difference of temperature appearing between the temperature of the sample and of the reference substance at the beginning of the phase transformation can be converted by a suitable converter into an electric control signal which can be used to interfere in the heating circuit of the furnace and to stop the increase of temperature. It has been recognized that said difference of temperature, since it appears at the beginning of the transformation, provides a possibility to interfere in the heating circuit of the furnace only at the beginning of the phase transformation.
In order that the recognition of the present invention may clearly be explained, previous solutions will be introduced somewhat more thoroughly. FIG. 1 shows among others a DTA apparatus as well as schematically a DTA-curve received thereby, FIG. 2 shows a DTA-curve. The known DTA apparatus comprises a furnace 3 the temperature of which is increased with a predetermined and uniform speed by a program-controller 4. A sample 1 and a reference substance 2 is placed in the furnace 3. Temperature of the sample 1 as well as that of the reference substance 2 are sensed by temperature sensing elements 5 and 6, respectively. The temperature sensing elements 5 and 6, which may preferably be thermocouples, are connected to each other in series opposition. The temperature sensing element 5 is coupled with a temperature measuring instrument 7 which may be e.g. a galvanometer and the signal of said instrument 7 measuring the temperature T of the sample 1 is applied to a recorder 10. The two temperature sensing elements 5 and 6 are also coupled with the recorder 10 through a temperature comparing unit 9 showing the temperature difference. The known DTA apparatus consequently comprises a furnace 3, a sample 1 and a reference substance 2, a program-controller 4, temperature sensing elements 5 and 6 as well as a temperature measuring instrument and a temperature comparing unit 9. Curves belonging to a known DTA apparatus are shown in FIG. 2 as follows:
2a--change of temperature of the reference substance 2, PA1 2b--change of temperature of the sample 1, PA1 2c--difference of temperature between the sample 1 and the reference substance 2 in an ideal case and in the case when there is no transformation in the sample 1. This is therefore an ideal reference or base-line line of the DTA-curve. PA1 2d--difference of temperature of the sample 1 and that of the reference substance 2 in real case and in case when there is no transformation in the sample 1. PA1 2e--DTA-curve indicating transformation, i.e. the signal measured at the output of the temperature comparing unit 9. PA1 2f--derivative of the curve 2c, i.e. d(2c)/dt, PA1 2g--derivative of the ideal reference line, i.e. EQU 2g=d(2e)/dt, PA1 2h--derivative of the displaced reference line, i.e. EQU 2h=d(2d)dt.
FIG. 1 shows that there is also a resistor 8 providing for the required symmetry in the measuring circuit. It is seen in FIG. 2 that the measured DTA-curve (curve 2e) shows how the reference line is displaced, i.e. how the temperature of the furnace 3 is changed also after the beginning of the transformation. The reference or base-line is the temperature value which does not change during thermal transformation. In FIG. 2, curve 2d shows that in the known DTA when thermal transformation begins the temperature is higher than when transformation has ended. The difference between the beginning and ending temperatures is the reference or base-line displacement. It is an obvious consequence that the transformations are not taking place in an ideal way but they generally end at a temperature which is 20-100 C..degree. higher than the real temperature of the transformation.
It is known from Hungarian Pat. No. 152 197 how a thermogravimetric curve may be plotted among isothermal circumstances. Heating is controlled by the speed of change of sample weight. Nevertheless, the displacement of the reference line does not occur in this case so that the possibility of interference is much more favourable. It appeared at first approach that a similar method can be used in case of DTA-curves plotted by a DTA apparatus such as that shown in FIG. 1 by means of a limit switch 11 and an actuating unit 12. We thought earlier that the program-controller 4 may be operated by the actuating unit 12 so that it becomes possible to interfere if the difference of temperature exceeds the threshold signal of the limit switch 11. This solution is not applicable because of the displacement of the curve 2d. There were made attempts earlier at increasing the accuracy of the temperatures of transformations.
Bean and Oliver describe a solution of this kind in UK Pat. No. 1 063 898 disclosing an electromechanical structure which prevents the speed of transformation from increasing if it exceeds a predetermined limit. They were forced to choose this limit too wide as a consequence of the above mentioned displacement of the reference line of DTA-curve. However, they did not made an attempt at eliminating the harmful effect of the reference line.
To FIG. 2 derivative functions also are shown as follows:
In this case, however, a limit value (2i) can be predetermined and sensed by a suitable limit switch as well as kept in a narrow range.